1. Field of the Invention
The present invention relates to a melt-processable polyimide resin composition and more particularly relates to an aromatic polyimide resin composition which has outstanding melt-processability and is excellent in heat and chemical resistances and mechanical strength and is particularly superior in repeated fatigue characteristics.
2. Description of the Related Art
Conventionally, polyimide has been known to provide excellent mechanical strength and dimensional stability in addition to its high heat resistance and has exhibited outstanding in flame retardance and electrical insulation. Consequently, polyimide has been used in the field of electric and electronic appliances, space and aeronautic equipment and transport machinery and is expected in future for wide use in the field where heat resistance is required. Polyimide of various kinds which exhibit excellent characteristics has conventionally been developed. However, such polyimide has no distinct glass transition temperature though outstanding in heat resistance and thus must be processed by such means as sintering in order to use as a molding material. On the other hand, polyimide which has excellent processability is insufficient in heat and solvent resistances because of a low glass transition temperature and solubility in halogenated hydrocarbon solvents.
Consequently, characteristics and drawbacks have been simultaneously found in conventional polyimide.
With the extended application of polyimide to various fields of use, it has been strongly desired to develop polyimide which can satisfy various kinds of performance.
In particular, in the above industrial field, it has been desired to develop a polyimide having excellent dynamic repeated fatigue characteristics in addition to the above inherent excellent properties of polyimides as an industrial material instead of a metal, for mechanical parts, automobile parts, for example, parts for internal combustion engine such as impeller of centrifugal compressor or, turbo-charger, parts for a silencer and exhaust system such as manifold, and valve-guides, valve-system, pistons, skirts, oil pans and the like.
In order to meet such demands, the present inventors have previously found melt-processable polyimide which is excellent in mechanical and electrical properties and also exhibits outstanding heat and chemical resistances, and has recurring structural units of the formula (1): ##STR3## wherein X is a radical selected from the group consisting of a direct bond, isopropylidene, hexafluoroisopropylidene, carbonyl, thio and sulfonyl, and R is a tetravalent radical having 2.about.27 carbon atoms and being selected from the group consisting of an aliphatic radical, alicyclic radical, monoaromatic radical, condensed polyaromatic radical and noncondensed aromatic radical connected to each other with a direct bond or a bridge member. (Japanese Laid-Open Patent Sho 61-143478, 62-86021, 62-205124, 62-235381 and 63-128025).
The polyimide is a melt-processable and heat-resistant resin having many excellent fundamental properties which are specific for polyimide. However, the polyimide is somewhat inferior in processability although heat resistance and other properties are much superior to engineering plastics such as polyethylene terephthalate, polyether sulfone and polyphenylene sulfide.
Another melt-processable polyimide having recurring structural units of the formula (5): ##STR4## and having a glass-transition temperature of about 200.degree. C. has been already disclosed in U.S. Pat. No. 4,485,140 and Japanese Laid-Open Patent SHO 61-143477.
A further polyimide having recurring structural units of the formula (6): ##STR5## and having high heat-resistance and very excellent melt flowability has been disclosed by the present inventors in Japanese Laid-Open Patent SHO 61-143477.
Still another polyimide having recurring structural units of the formula (7): ##STR6## and having extremely excellent melt-flowability has been disclosed by the present inventors in Japanese Laid-Open Patent SHO 62-50375.
Further, polyether pyridine having recurring structural units of the formula (4) ##STR7## wherein Y is a divalent radical represented by ##STR8## and having high heat-resistance and extremely outstanding melt-flowability has also been found by the present inventors (Japanese Laid-Open Patent SHO 63-37123).
Some of polyether pyridines have a high melting point of 250.degree. C. or above, initiate melt-flow in the neighborhood of 270.degree. C., have a very low viscosity in the molten state and are thus excellent in melt-processability.
Although polyimide and polyether pyridine have good melt-processability, other characteristics are still unsatisfactory and cannot meet the demand for polyimide which is excellent in the above various performances and has good processability.
Further, a polyimide instead of a metal, which has excellent dynamic repeated fatigue characteristic has not been disclosed.